Tunnelblick is a program that can be used to securely connect a Mac running OS X to an OpenVPN server. The server then connects the Mac to a remote network or to the Internet, bypassing untrusted networks, censorship, and eavesdropping.

It does this by creating a "Virtual Private Network", or "VPN", to the OpenVPN server using a program named "OpenVPN", which is included within the Tunnelblick application. When you connect through a VPN, your computer sends some or all network traffic through a "tunnel" to the VPN server, which then passes on your network traffic to a local network or the Internet. It is as if you were connecting to the network or Internet through the VPN server instead of your computer. Normally, all traffic between your computer and the VPN server is encrypted.

In addition to Tunnelblick, you need access to a VPN server. Your company may provide one, or you can obtain VPN service from any of several VPN service providers, or you can use another one of your computers or a router to act as a VPN server.

Tunnelblick is also a ready-to-use Graphic User Interface (GUI) for OpenVPN on Mac OS X. It provides easy-to-use control of OpenVPN server and/or client connections.